Stabilizing Windings

Author: E.E. Kimberly

To insure sparkless commutation, many motors are designed with their interpole fluxes quite strong in comparison with the main-pole fluxes. When such motors are operated at their maximum speeds, and hence with their main poles only weakly excited, the distortion of the main flux and the resulting saturation of the tips of the trailing poles decrease the total flux and the net counter emf so seriously that unstable operation results.

Fig. 11-5. Effect of Stabilizing Winding

Fig. 11-6. Effect of Compensating Winding

To prevent such instability, a series winding composed of a very few turns of large conductor is added to each main pole and is so connected that its mmf is in the same direction as the mmf of the shunt-field winding. Such a winding is called a stabilizing winding because its flux, which is added in the same relative amount as that lost because of pole-tip saturation due to "crowding," prevents field weakening and stabilizes the speed. Fig. 11-5 (c) shows the effect of a stabilizing winding. Many so-called shunt motors are provided with stabilizing windings.